1. [Technical Field of the Invention]
The present invention relates to a production process of a quartz glass crucible by a ring-like arc fusion, in which the process is suitable as a production of a crucible having a large open diameter, and also relates to its quartz glass crucible.
2. [Discussion of the Background]
The production process of the quartz glass crucible according to a rotational mould method is the process, in which a quartz powder being stored in the inside of a rotational mould is heated to be fused by an arc discharge to produce the quartz glass crucible having the transparent layer at the inside and the opaque layer at the outside. As for the conventional structure of the electrode forming the arc discharge, 3 electrodes are used, and the 3 phases alternating electric current flows to said 3 electrodes to form the arc (discharge) plasma between each of the electrodes.
[Problems to be Solved]
According to the enlargement of the quartz glass crucible, the electrode structure, which can form the arc discharge having a large heating range, has been required. The conventional main electrode structure has 3 electrodes to 3 phases alternating current, and this structure has the fault that the arc becomes unstable to be cut off when a distance between the electrodes is increased in order to expand the heating range. Especially, as for the large size crucible, the influence of air stream at the inside of the crucible by the rotation of the mould becomes large, so that the arc is cut off easily with the conventional electrode structure.
Therefore, trials increasing the number of the electrode to expand the heating range, have been carried out, and the structure having 6 electrodes to 6 phases alternating current is proposed. However, as for the structure having 6 electrodes to 6 phases current, as shown in FIG. 6, it is easy to generate arc discharge between the electrodes facing each other rather than between the electrodes neighboring each other, so that there is a problem that the heat of discharge at the central portion being encircled with the electrodes becomes large excessively rather than the heat at the around portion, so that it is difficult to heat uniformly the inside of the crucible.
The present invention solves said conventional problems of the electrode structure in the production device of the quartz glass crucible, and provides the electrode structure forming the ring-like arc, which is stable and has a large heating range, and is suitable to the production of a crucible having a large open diameter. Moreover the present invention provides the production process of the quartz glass crucible by using said electrode structure, and its quartz glass crucible.